Highly crack resistant glasses are used for cover glasses/displays in electric devices, both portable and stationary. Typically, the high crack resistance is achieved through the ion-exchange of alkali containing glasses, such as soda-lime silicates, alkali boroaluminosilicates, and alkali aluminosilicates (e.g., Corning® Gorilla® glasses such as Code 2317 and Code 2318). The small alkali ions within the glass (e.g., lithium, sodium, or both) are replaced by a larger alkali (potassium) ions, and on cooling to room temperature the exchanged layers on either face of the sheet exhibit substantial compressive stress, while the interior of the sheet exhibits tensile stress. The compressive stress serves as a barrier, in effect, to crack initiation and propagation. In particular, the compressive stress thwarts formation of median/radial and lateral cracks that propagate away from the locus of an initial flaw. Median/radial cracks are of particular interest since their orientation is perpendicular to the surface of the glass and thus act as strength limiting flaws during bending of a glass sheet.
One measure of crack resistance is the load required to generate median/radial cracks using a Vicker's diamond indenter. For example, Corning® Gorilla® glass Code 2317 before ion exchange requires about 300 grams of force (go to produce median/radial cracks using a Vickers diamond indenter tip, while after ion exchange, the force required to produce median/radial cracks increases to approximately 5000 grams. This sizeable improvement in the load required to produce median/radial cracks is a key reason why Code Corning® Gorilla® glass 2317 is finding increasing use in consumer electronics applications.
While highly desirable for ion-exchange performance, alkalis are detrimental to certain consumer electronics applications, particularly those in which the glass is a substrate for use in semi-conductor based electronics, such as a display or integrated touch applications. This is because alkalis can move into the semiconductor layers, impacting switching voltages. Furthermore, as sheets become thinner, the maximum compressive stress that can be obtained for a given compressive layer thickness diminishes because of strain through the thickness of the glass. In addition, ion exchange must be performed on each part obtained from a sheet, rather than on the entire sheet before creating parts, because division of an ion exchanged sheet into two or more parts exposes the region under central tension and severely compromise the strength of the edge. Ion exchange can be time consuming and expensive, and in applications requiring large numbers of parts, ion exchange capacity can severely limit overall throughput of finished parts. Finally, in practical terms the amount of compressive stress that can be installed via ion exchange is not arbitrarily large. This is because once the central tension for a given glass exceeds a particular value, a flaw that penetrates through the layer of compressive stress causes an abrupt and catastrophic release of energy, causing the part to break violently into many pieces. This maximum level of installed central tension is referred to as the frangibility limit. Exceeding the frangibility limit must be avoided at all cost for cover glass and integrated touch applications.
For these reasons, it is desirable to identify compositions that are intrinsically resistant to formation of cracks, meaning without requiring ion exchange. It is further desired that such glasses be substantially free of alkalis so that they can be freely used as substrates for electronics applications. It is also desirable that the glasses possess viscoelastic properties, molar volumes, coefficients of thermal expansion, durabilities, etc. such that they can be used in typical consumer electronics applications, e.g., as cover plates or as substrates for integrated touch applications. Such glasses would have the additional advantage relative to ion-exchanged glasses in that they are intrinsically nonfrangible.